Talk about a coincidence! No sooner had I penned my Autumn of the Automakers piece, arguing that Ford's (NYSE:F) decision to slash fourth-quarter production targets (a decision that General Motors (NYSE:GM) has also taken) boded well for used car hawkers such as CarMax (NYSE:KMX), than the latter company's long period of declining stock prices ended, and an enormous run-up ensued. In fact, since the opening bell rang on Wall Street on Sept. 2, CarMax shares have now gained an incredible 49.5% against a measly 6.1% gain for the S&P 500.

Yesterday, CarMax received its latest boost toward that total 49.5% return, in the form of a 6.6% spurt upward in response to the company's announcement that its previous pessimism had proven unfounded. Back in late September, CarMax warned investors that its same-store sales would likely decline in the third quarter by somewhere between 2% and 8%, resulting in earnings of just $0.12 to $0.17. In point of fact, comparable store sales have actually increased, and by the time earnings come out officially, they look likely to hit a 2-3% gain. Likewise with earnings, which should at least max out the predicted earnings range and quite possibly beat it by a penny.

At this point in the run-up, it's high time we pause and take a look at how CarMax stacks up against the competition. We'll look at CarMax and four of its closest publicly traded competitors, comparing their enterprise values to their efficiency of cash generation, as expressed by their respective returns on investment (ROE):

EV & FCF numbers in millions of $
Enterprise value Free cash flow EV/FCF ROE
Auto Nation (NYSE:AN) 7900 323 24 9%
Sonic Automotive (NYSE:SAH) 2600 61 43 14%
America's Car-Mart (NASDAQ:CRMT) 300 2 150 20%
United Auto Group (NYSE:UAG) 3100 (19) - 11%
CarMax 2900 (119) - 15%

Judging from the above data, CarMax actually appears to be the least attractive of all five of these unattractively priced companies. In retrospect, though, it shouldn't be surprising that these companies need some bodywork done. Years of getting financially "keyed" by the giveaway prices Detroit is slapping on new autos have done a real number on their cash flows. Meanwhile, their stock prices have run up steeply in anticipation of free cash flow improvement. It's going to take some more time for the FCF and earnings numbers to rise in support of the stock prices. And until that happens, it's really anyone's guess whether the stock price run-ups should continue or stall out.

For more Foolish news and views on the nation's car sellers, read:

Fool contributor Rich Smith owns no shares in any of the companies mentioned in this article.